Airfoils may have airstream separation problems either at low or at high speeds. Airfoil stall occurs at low airspeeds (Mach numbers below about 0.15) when the airstream no longer can follow the airfoil contour and separates therefrom. Shock separation occurs at higher speeds (Mach numbers of, say, about 0.6 to about 0.9) when the airstream near the airfoil accelerates to supersonic speed and then decelerates suddenly at some location forward of the airfoil trailing edge. This deceleration is accompanied by sudden pressure rises and sometimes by airstream separation, which may excite airframe structure or movable surfaces such as ailerons, producing what are known as "buffeting" and, eventually, aileron "buzz".
Vortex generators are blade-like devices attached to upper or lower airfoil surfaces at various angles to generate turbulent flow in an airstream boundary layer of an airfoil. Vortex generators are known expedients to generate turbulent flow in an airstream boundary layer of an airfoil at either high or low speeds. The vortex generators extend the onset of separation by protruding from about 0.6 to about 1.1 of the airstream boundary layer to redirect the airstream over the airfoil for reattachment. For example, the vortex generators will reduce the onset of aileron buzz when placed upstream on the upper surface of an airfoil. Vortex generators may also be placed near the root section of a wing to maintain airflow attachment to an airfoil at low air speeds.
While vortex generators and other similar blade-type devices are used to create such turbulent flow, they have associated problems of creating high drag at high air speeds because of their large projection in relation to the boundary layer. The large projections make them susceptible to damage or loss through inadvertent actions of maintenance personnel.